The present invention relates to a filter medium in th form of a hollow fiber. More particularly, to a hollow fiber filter medium having a large pore diameter made of a polysulfone resin and a process for the preparation thereof.
Hollow fiber filter mediums have the following advantages. They enable a filter to be made in a small size with a simple structure. The flow of a fluid through the filter becomes uniform, since the effective membrane area per unit volume can be made large as compared with flat filter mediums. Therefore, in recent years, porous hollow fibers are being utilized as filter mediums in various fields in place of the flat filter membrane.
Among the known hollow fiber filter mediums are hollow fibers made from resins such as polyvinyl alcohol, cellulose acetate, polymethyl methacrylate, polypropylene and polyethylene. These are known as hollow fiber filter mediums having a large pore diameter. The hollow fiber filter mediums made from these resins are not always satisfactory both in performance and preparation method thereof. For instance, performance is undesirable that steam sterilization requiring high temperatures is impossible because of a lack in heat resistance. Also the rate of filtration is low and the chemical resistance, mechanical strength or affinity for a living body is insufficient. Also, the methods for the preparation thereof have the defects that because of using high molecular weight compounds as pore-forming agents, a long time is required for removing them by extraction. It is also necessary, therefor, to use toxic solvents.
A polysulfone resin is superior in heat resistance, mechanical resistance, chemical resistance and affinity for a living body. It has been utilized as a support for ultrafiltration membrane and reverse osmosis membrane, and many literatures describing such uses have been published. However, a polysulfone hollow fiber filter medium having a large pore diameter, at least having pores or openings of 0.01 to several micronmeters in diameter in the inner and outer surfaces, cannot be prepared by a conventional technique. Therefore, no polysulfone hollow fiber filter medium having pores or openings of not less than 0.01 .mu.m. in diameter at least in the inner and outer surfaces has been provided up to now. For instance, processes for preparing polysulfone hollow fibers are reported in Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 16378/1979, No. 143777/1979, No. 145379/1979, No. 152704/1981 and No. 83515/1982; and Journal of Applied Polymer Science, Vol. 20, 2377-2394(1976), Vol. 21, 165-180(1977) and Vol. 21, 1883-1900(1977). These publications disclose processes for preparing hollow fibers having a so-called asymmetric structure. That is, hollow fibers substantially having a thin, dense layer containing pores of less than 0.01 .mu.m. in diameter in either one or both of the inner surface and the outer surface thereof. The amount of water permeation of these hollow fibers is small, even if the dense layer is made as thin as possible.
It is an object of the present invention to provide a porous hollow fiber having a large pore diameter which is useful as a filter medium.
A further object of the invention is to provide a hollow fiber filter medium having a large pore diameter and which is excellent in heat resistance, mechanical strength, chemical resistance and affinity for a living body.
Another object of the invention is to provide a process for preparing a porous hollow fiber useful as a filter medium.
These and other objects of the present invention will become apparent from the description hereinafter.